Migrate block 39 language/async_backwards ... /async_star_take.

tests/language_strong/async_helper_lib.dart

Index: tests/language_strong/async_helper_lib.dart
diff --git a/tests/language_strong/async_helper_lib.dart b/tests/language_strong/async_helper_lib.dart
deleted file mode 100644
index e91efac553840541e49cd8d2fe8fb904738fe42c..0000000000000000000000000000000000000000
--- a/tests/language_strong/async_helper_lib.dart
+++ /dev/null
@@ -1,7 +0,0 @@
-// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-library async;
-
-class async {}